It was shown that the uracil nucleotide content of heart and skeletal muscle was significantly lower in streptozotocin-diabetic and fasted rats than in normal fed controls. Parallel changes in the activity of uridine kinase in cytosolic extracts of muscle have been found. Insulin treatment of diabetic rats will reverse the effects of diabetes on uridine kinase activity and uracil nucleotide content of muscle (Gertz, B. J. and Haugaard, E. S., 1979, Metabolism 28, 358). Insulin in vitro was found to increase the UTP content of rat diaphragm (Haugaard, E. S., Frantz, K. and Haugaard, N., 1977, PNAS 74, 2339). Investigation of the mechanism(s) of activation of uridine kinase by insulin, both in vivo and in vitro are planned. We find that the uridine kinase activity of muscle and liver is increased by thyroid hormone and that actinomycin D blocks the effect of T3 on uridine kinase of heart. Similar studies are planned on the effect of adrenal cortical steroids on uridine kinase activity of muscle.